don't use AsFractional backwards and remove AsFractional → Fractional instance

The combine_sep_fractional_bwd instance for CombineSepAs uses AsFractional in a backwards way: given a predicate Φ and a fraction q, compute the P that is equivalent to Φ q. However this is not really how the AsFractional typeclass is designed to be used, and beta reduction should take care of simplifying Φ q if necessary.

Also the as_fractional_fractional instance was only meant to be used internally in this file; let's see if we can get away without it.